Greenhouse gases (GHGs) control energy flows in the atmosphere by absorbing infra­red
radiation emitted by the earth. They act like a blanket to keep the earth¡¯s surface
some 20¢ªC warmer than it would be if the atmosphere contained only oxygen and nitrogen.
The trace gases that cause this natural greenhouse effect comprise less than 1% of the atmosphere.
Their levels are determined by a balance between "sources" and "sinks". Sources
are processes that generate greenhouse gases; sinks are processes that destroy or remove them. Apart
from industrial chemicals like CFCs and HFCs, greenhouse gases have been present naturally in the
atmosphere for millions of years. Humans however, are affecting greenhouse gas levels by introducing
new sources or by interfering with natural sinks.

The largest contributor to the natural greenhouse effect is water vapour. Its presence in the
atmosphere is not directly affected by human activity. Nevertheless, water vapour matters for climate
change because of an important "positive feedback". Warmer air can hold more moisture, and
models predict that a small global warming would lead to a rise in global water vapour levels,
further adding to the enhanced greenhouse effect. On the other hand, it is possible that some regions
may become drier Because modeling climate processes involving clouds and rainfall is particularly
difficult, the exact size of this crucial feedback remains unknownuncertain.

Carbon dioxide is currently responsible for over 60% of the "enhanced" greenhouse
effect. This gas occurs naturally in the atmosphere, but burning coal, oil, and natural gas is
releasing the carbon stored in these "fossil fuels" at an unprecedented rate. Likewise,
deforestation releases carbon stored in trees. Current annual emissions amount to over 23 billion
metric tons of carbon dioxide, or almost 1% of the total mass of carbon dioxide in the
atmosphere.

Carbon dioxide produced by human activity enters the natural carbon cycle. Many billions of
tonnes of carbon are exchanged naturally each year between the atmosphere, the oceans, and land
vegetation. The exchanges in this massive and complex natural system are precisely balanced; carbon
dioxide levels appear to have varied by less than 10% during the 10,000 years before
industrialization. In the 200 years since 1800, however, levels have risen by over 30%. Even with
half of humanity's carbon dioxide emissions being absorbed by the oceans and land vegetation,
atmospheric levels continue to rise by over 10% every 20 years.

A second important human influence on climate is aerosols. These clouds of microscopic
particles are not a greenhouse gas. In addition to various natural sources, they are produced from
sulphur dioxide emitted mainly by power stations, and by the smoke from deforestation and the burning
of crop wastes. Aerosols settle out of the air after only a few days, but they are emitted in such
massive quantities that they have a substantial impact on climate.

Most aerosols cool the climate locally by scattering sunlight back into space and by affecting
clouds. Aerosol particles can block sunlight directly and also provide "seeds" for
clouds to form, and often these clouds also have a cooling effect. Over heavily industrialized
regions, aerosol cooling may counteract nearly all of the warming effect of greenhouse gas increases
to date.

Methane levels have already increased by a factor of two and a half during the industrial era.
The main "new" sources of this powerful greenhouse gas are agricultural, notably flooded
rice paddies and expanding herds of cattle. Emissions from waste dumps and leaks from coal mining and
natural gas production also contribute. The main sink for methane Methane is removed from the
atmosphere by chemical reactions in the atmosphere that are very difficult to model and
predict.

Methane from past emissions currently contributes 20% of the enhanced greenhouse effect. The
rapid rise in methane started more recently than the rise in carbon dioxide, but methane's
contribution has been catching up fast. However, methane has an effective atmospheric lifetime of
only 12 years, whereas carbon dioxide survives much longer.

Nitrous oxide, a number of industrial gases, and ozone contribute the remaining 20% of the
enhanced greenhouse effect. Nitrous oxide levels have risen by 16%, mainly due to more intensive
agriculture. While chlorofluorocarbons (CFCs) are stabilizing due to emission controls introduced
under the Montreal Protocol to protect the stratospheric ozone layer, levels of long-lived gases such
as HFCs, PFCs and sulphur hexafluoride are increasing. Ozone levels are rising in some regions in the
lower atmosphere due to air pollution, even as they decline in the stratosphere.

Humanity's greenhouse gas emissions have already disturbed the global energy budget by about
2.5 Watts per square metre. This equals about one percent of the net incoming solar energy that
drives the climate system. One percent may not sound like much, but added up over the earth's
entire surface, it amounts to the energy content of released by burning 1.8 million tonnes of oil
every minute, or over 100 times the world's current rate of commercial energy consumption. Since
greenhouse gases are only a by-product of energy consumption, it is ironic that the amount of energy
humanity actually uses is tiny compared to the impact of greenhouse gases on natural energy flows in
the climate system.